Wednesday, November 11, 2015: 1:50 PM
355B (Salt Palace Convention Center)
The water-gas shift reaction (WGS) is an important reaction for hydrogen production and fuel cell applications1, which are important for renewable energy. Au supported on oxides has been identified as an active catalyst for WGS. Supports are critical for H2O dissociation and play an important role in determining the WGS rates and kinetics. However, the participation of hydroxyl groups in the WGS reaction mechanism is still not well understood. In the present work, non-porous nano MgO (~200 m2/g) and Mg(OH)2 (~70 m2/g) were adopted as supports and loaded with 2.5 wt% Au. WGS rates and kinetics were measured on these catalysts. Au/MgO and Au/Mg(OH)2 showed similar kinetics except for the apparent order with respect to H2O. Thus, these two systems provide one way to study the hydroxyl group influences on the WGS reaction. A lower apparent order with respect to H2O was observed for Au/MgO (0.7±0.1) than for Au/Mg(OH)2 (1.0±0.1). This implies a higher H2O/OH coverage over the Au/MgO compared with Au/Mg(OH)2, which corresponds to a higher binding affinity for H2O/OH on Au/MgO. Both catalysts showed similar WGS rates per mole of Au (1.5±0.3x10-3(mol H2)(mol Au)-1s-1 at 220 oC). TEM was performed on both catalysts to determine Au particle sizes. Au/MgO showed a number-averaged Au diameter 4.0±0.9 nm while Au/Mg(OH)2 showed a corresponding Au diameter of 2.2±0.7 nm. Since WGS rate decreases significantly as Au particle size increases2, Au/MgO is deduced to be a more active catalyst for WGS at the same Au particle size. A kinetic isotope effect (KIE), which is the ratio between the WGS rate with H2/H2O and WGS rate with D2/D2O, was measured for both catalysts and both showed the same KIE ratio of about 2.0±0.3. This similar KIE implies a similar reaction mechanism on both catalysts and that breaking of a covalent hydrogen bond is involved in the rate-determining step. Density Functional Theory (DFT) calculations also revealed a decrease of about 0.7 eV in the energy barrier for H2O dissociation at Au/MgO interface compared with pure MgO and pure Au. Further experimental studies on other supports such as TiO2, ZrO2, Al2O3 etc. also shows that a lower apparent order with respect to H2O (about -0.3) results in a higher WGS rate (~2x10-2(mol H2)(mol Au)-1s-1 at Au nanoparticle size ~2nm). In summary, the importance of H2O dissociation in WGS activity is confirmed.